This invention relates to an electrophotographic photoreceptor, and more particularly the invention relates to an improved dual-layered photoreceptor employing vitreous selenium as a photosensitizing layer and a polymeric carbazole derivative as a charge-retaining and charge-transport layer. The photoreceptor has a long shelf-life with high stability in repeated use.
Conventionally, it is known that a dual-layered photoreceptor can basically be formed by overlaying a thin vitreous selenium layer on an electrically conductive support either directly or indirectly through an electrical barrier layer and then overlaying a relatively thick top layer of an electron-doner type organic polymeric carbazole derivative, such as poly-N-vinyl carbazole, which is substantially non-light sensitive in the visible ray region, but exhibits the ability to transport the charge carriers generated in said adjacent selenium layer, when subjected to an actinic light.
The detailed descriptions of the prior art concerned with the dual-layered photoreceptor are found, for example, in U.S. Pat. No. 3,725,058 and 3,850,629, South African Pat. No. 7,101,218 and Preprint of Annual Conference: Society of Photographic Scientists and Engineers, page 116 to 117, June 1972.
The polymeric carbazole derivative (hereinafter conveniently called "polyvinyl carbazole") referred to herein, is designated as a polymer of vinyl carbazole and/or its derivative, or a copolymer of N-vinyl carbazole or its derivative and another vinyl compound, such as vinyl acetate and methyl methacrylate. The derivative referred to herein has a substituent, such as a halogen atom, nitro radical, alkyl aryl radical, amino radical or alkylamino radical, in place of a hydrogen atom in a carbazole ring in the recurring unit of the above mentioned polymers, as shown in the following chemical formula: ##STR2## wherein, X is the substituent. The number and position of said substituents in the carbazole ring and the polymerization degree of the resultant polymer are determined according to the method for preparing them and are not limited to those expressively disclosed herein.
It is known that the polyvinyl carbazole used in this dual-layered photoreceptor does not necessarily contain a Lewis acid and/or a sensitizing dye, because the polyvinyl carbazole layer is optically sensitized by the said selenium layer intimately attached thereto. On the contrary, addition of sensitizers sometimes causes a disadvantage for the completed dual-layered photoreceptor for the reason that some well-known sensitizers effective to sensitize a polyvinyl carbazole single layer photoreceptor (cf U.S. Pat. Nos. 3,037,861 and 3,484,237) act as light filters for the selenium layer or act as charge-traps for the present polyvinyl carbazole charge-transport layer. However, when the polyvinyl carbazole layer requires plasticizers and/or film-forming binders to increase flexibility and film-strength, addition of some specified active additives are sometimes effective in order to prevent an increase of residual voltage of the completed dual-layered photoreceptor in repeated use.
These improvements concerned with the dual-layered photoreceptor are described in the aforesaid U.S. Pat. No. 3,850,629.
The operable thickness of the selenium layer, as taught by the aforesaid U.S. Pat. No. 3,725,058, is from 0.05 to 3 .mu.. A selenium layer thinner than 0.05 .mu. shows a marked reduction in the photosensitivity of the completed dual-layered photoreceptor. A selenium layer thicker than 3 .mu. shows a relatively high residual voltage and brittleness.
It has been known that metallic aluminum is the most preferable material for an electrode of an electrophotographic photoreceptor employing vitreous selenium, because it is easy to fabricate at a low cost, and further because the completed photoreceptor shows a high chargeability in the dark, due to a barrier layer produced between the aluminum and the selenium. Although the metallic aluminum is an advantageous material for electrodes, as described above, it sometimes injures the adjacent vitreous selenium layer as time passes, if the selenium layer is thinner than 3 .mu.. The following is an example of the prior art described above.
A vitreous selenium layer of 0.5 .mu. in thickness is vapor coated on a 500 A thick layer of metallic aluminum, which has also been vapor-coated on a plastic substrate in the same vacuum jar, prior to coating said selenium without breaking the vacuum. The surface of the vitreous selenium layer changes its original reddish brown color to black in less than five hours. A completed dual-layered photoreceptor using such a black selenium does not show a normal photosensitivity any longer.
The X-ray analysis of such a black selenium shows that it belongs to the hexagonal system. When the vitreous selenium layer, and overlaying aluminum layer, is placed in air containing a humidity of 60% RH or more, tiny spots of a tree-like white crystal come out while the vitreous selenium gradually changes its reddish brown color to black. These white crystals grow as time passes, and finally, some of them form circular or fan-shape white areas of 10 to 20 mm in diameter. These seem to form spherulites.
From an electron diffraction analysis, the white crystalline material is regarded as a composite consisting of a very small amount of selenium oxide and a large amount of selenium that orients, although the degree of crystallization is not very high. The changeability of the original vitreous selenium into crystalline form is likely to be induced by the crystallization of the vapor-deposited aluminum layer. That is, the speed of crystallization in the selenium layer increases in accordance with an increase in the degree of crystallization in the aluminum layer.
If a vapor-deposited aluminum layer that has been placed for a short time in atmospheric air before selenium is coated, the overlaid vitreous selenium changes its reddish brown color into black within one day, when it is at room temperature. The white spots also come out and tend to grow when the room is more humid than 60% RH.
A conventional metallic aluminum foil, fabricated through a rolling process at high temperature from 500.degree. C. to 600.degree. C., forms a relatively thicker oxide layer on its surface, as compared with those of vacuum coated aluminum layers. When a vitreous selenium layer thinner than 3 .mu. is vacuum-coated on the conventional metallic aluminum foil, it partially changes its reddish brown color to black in one to two months at 30.degree. C. and in a humidity of more than 70% RH. The completed dual-layered photoreceptor using such a black selenium on the foil provides not only the absence of electrophotographic photosensitivity but also useless adhesion to the aluminum foil. Actually, the adhesion of the fresh selenium to the aluminum foil is essentially so weak that it seems to be at its lowest level for practical purposes in the reused photoreceptor. The crystallization reduces the adhesion from the lowest sufficient level to an insufficient level.
In addition, a completed dual-layered photoreceptor using a vitreous selenium which has such a black area or white spots, provides undesirable image noise expressed as background fog or white dots, when toned by ordinary electrophotographic imaging method such as magnetic brush development or liquid development. Even if the area or spot of the photoreceptor is barely visible to the naked eye, the corresponding toned noises are clearly distinguishable.
Thus, it can be said that the selenium layer, even if the thickness thereof is less than 3 .mu., does not adhere well to a metallic aluminum surface in general. In order to improve such a disadvantage, the surface of the aluminum is, according to prior art, roughened with mechanical scribing, chemical etching and electrochemical etching, or cleaned by a physical method such as ionic bombardment and electron-beam bombardment. This treatment of the metallic aluminum surface as above requires equipment and man-power which will increase expense of the completed dual-layered photoreceptor. In the art of xerography, a single vitreous selenium layer of thickness of from 20 .mu. to 50 .mu. which is mechanically brittle is commonly used as a xerographic photoreceptor. To prevent crack generation in the selenium layer, which relates to the layer separation from the substrate, the xerography employs a thick and rigid plate or drum of metallic aluminum as a substrate. In spite of employing such a rigid aluminum substrate in xerography, there have been many attempts to improve the adhesion strength between the xerographic vitreous selenium and the rigid aluminum substrate, such as forming an oxide layer by chemical or physical treatment, or pre-coating a thin layer of a different material before coating selenium. The reason for requiring such a pre-treatment of the xerographic aluminum may be (1) to increase adhesion strength and (2) to prevent crystallization or chemical change of the vitreous selenium.
As mentioned in the foregoing example, pure and fresh metallic aluminum has a general tendency to promote the crystallization or chemical change of the vitreous selenium intimately attached thereto.
Especially in the case of a selenium layer thinner than 3 .mu. such as a layer used in the dual-layered photoreceptor in the present invention, adhesion between the vitreous selenium and a substrate is a more serious problem than with the xerographic rigid plate, because the present dual-layered photoreceptor is largely characterized by its flexibility. Thus, the xerographic aluminum plate or drum that is the most commonly useful material for a photoreceptor employing vitreous selenium is still on unsatisfactory material for the present dual-layered photoreceptor.
The summary of the above description is that, when metallic aluminum is employed as an electrode material of a dual layered photoreceptor by overlaying a vitreous selenium layer directly and further overlaying a polyvinylcarbazole layer thereon, is that (1) it plays the role of a catalyst to promote a physical and/or chemical change of the adjacent selenium layer and (2) it provides unsatisfactory adhesion to the adjacent selenium layer.
If such a dual-layered photoreceptor is reused many times in an electrophotographic office copying machine, it may easily be damaged by miscellaneous factors such as light, electrical field, corona ion, heat, mechanical stress, humidity and if used, a solvent. Some of these factors are continuously and some are intermittently applied to the dual-layered photoreceptor.
Conducting materials other than aluminum have been also reported to serve as an electrode of an electrophotographic photoreceptor employing vitreous selenium. Known materials include (1) a metallic material such as copper, zinc, nickel, iron, chromium, indium, tin, titanium silver, gold and a metallic alloy thereof and (2) inorganic semiconductive materials such as copper iodide and tin oxide.
However, they are rarely useful for the present dual-layered photoreceptor because of at least one of the following reasons. They:
(1) show a weak adhesion between the overlaid selenium and the said material, (2) provide a poor chargeability to the completed dual-layered photoreceptor, (3) acting as a catalyst for a physical or chemical change of the overlaid selenium and (4) react with the overlaid selenium on aging.